In recent years, such terminals as car navigation devices, portable phones, PDAs (Portable Digital Assistants), etc. have been enhanced rapidly, so that mass of data have also been handled in those devices. Under such circumstances, mobile hosts have come to be provided with a database management system respectively to cope with management of such mass of data efficiently. The database of each of such mobile hosts stores part or whole of data items of a database held by its fixed service host (hereinafter, to be referred to as a “service host”). For example, in a map distribution system for in-vehicle devices, the in-vehicle devices store the data items of a map database managed by the map center (service host).
However, if some data items in the database held by the service host are updated, the updated data items lose the consistency with the data items in the database held by the mobile host, thereby the user might access invalid data items. In such a case, the mobile host communicates with the service host to acquire updated data items and update the data items in its own database.
Updating the database held by the mobile host requires much time if there are many data items to be updated, since the wireless communication between the mobile host and the service host is low-speed and furthermore, the mobile host hardware performance is low. One of the effective methods for avoiding such a problem is incremental update method, which updates only the data items required by the user. In the case of this incremental update method, the mobile host updates only part of data items in its own database. For example, JP-A No. 2006-293696 discloses a technique of an in-vehicle data update system. According to the technique, the in-vehicle device acquires only the latest POI (Point of Interest)s included in a user's desired region from the map center. At this time, the map center sets the maximum number of POIs to be sent to the in-vehicle device. The communication load is prevented from increasing when updating is made for a region in which the number of POIs has increased abruptly because the number of data to be updated is limited.